Data communication method and mobile communication system

ABSTRACT

Since OFDM (Orthogonal Frequency Division Multiplexing), which is a downlink access scheme used for an LTE (Long Term Evolution) communication system, is weak at interference, it is preferable that the number of base stations for transmitting the same E-MBMS (Evolved Multimedia Broadcast Multicast Service) data be not excessive. 
     Thus, the mobile terminal performs: processing of obtaining measurement quality values by measuring reception quality of each base station; processing of deciding on whether the measurement quality value of a serving base station exceeds a certain receiving level; processing of adding measurement quality values of other base stations to the measurement quality value of the serving base station until the sum exceeds the certain receiving level; and processing of notifying the serving base station of the serving base station and other base stations as candidates for an active set for E-MBMS. This makes it possible to select as the active set for E-MBMS an appropriate number of base stations that make it compatible to maintain good reception quality and to prevent interference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/278,357 filed Aug. 5, 2008, the entire contents of which isincorporated herein by reference. U.S. application Ser. No. 12/278,357is a National Stage of Application No. PCT/JP06/319015 filed Sep. 26,2006.

TECHNICAL FIELD

The present invention relates to a mobile communication system in whicha base station performs radio communication with a plurality of mobileterminals, and particularly to a data communication method and a mobilecommunication system for providing multimedia multicast/broadcastservice to the mobile terminals.

BACKGROUND ART

Among the so-called third generation communication schemes, commercialservice of a W-CDMA (Wideband Code division Multiple Access) scheme hasstarted from 2001 in Japan. In addition, HSDPA (High Speed DownlinkPacket Access) service is scheduled to start which implements higherspeed data transmission using a downlink by adding a channel (HS-DSCH:High Speed-Downlink Shared Channel) for packet transmission to thedownlink (dedicated data channel and dedicated control channel).Furthermore, to further speed up uplink data transmission, an HSUPA(High Speed Uplink Packet Access) scheme has been proposed andinvestigated. The W-CDMA is a communication scheme determined by 3GPP(3rd Generation Partnership Project), a standardization organization ofmobile communication systems, and specifications of a sixth releaseversion have been arranged at present.

In addition, as communication schemes other than W-CDMA, 3GPPinvestigates new communication schemes referred to as “Long TermEvolution” (LTE) for radio sections and “System Architecture Evolution”(SAE) for the total system configuration including a core network. InLTE, an access scheme, radio channel configuration and protocol differfrom those of the current W-CDMA (HSDPA/HSUPA). For example, as for theaccess scheme, although W-CDMA employs CDMA (Code Division MultipleAccess), LTE uses OFDM (Orthogonal Frequency Division Multiplexing) inthe downlink direction, and SC-FDMA (Single Carrier Frequency DivisionMultiple Access) in the uplink direction. In addition, as for abandwidth, although W-CDMA is 5 MHz, LTE can use 1.25/2.5/5/10/15/20MHz. Besides, LTE does not employ circuit switching as W-CDMA does, butuses only a packet communication scheme.

Since LTE employs a new core network different from the core network(GPRS) of W-CDMA to construct a communication system, it is defined as aradio access network independent of a W-CDMA network. Accordingly, todistinguish from the W-CDMA communication system, in the LTEcommunication system, a base station for performing communication withmobile terminal UE (User Equipment) is called eNB (E-UTRAN NodeB), and abase station controller (Radio Network Controller) for transferringcontrol data and user data between it and a plurality of base stationsis called an aGW (Access Gateway). The multimedia multicast/broadcastservice carried out by the LTE communication system is referred to asE-MBMS (Evolved Multimedia Broadcast Multicast Service), and transmitsmasses of broadcasting mass contents such as news, weather forecasts andmobile broadcasting to a plurality of mobile terminals. It is alsoreferred to as point to multipoint service. A base station transmitsE-MBMS data to mobile terminals by mapping the E-MBMS data on a DL-SCH(Downlink Shared Channel) or on an MCH (Multicast Channel). In addition,LTE provides not only broadcast communication service, but alsocommunication service to each of the plurality of mobile terminals. Thecommunication service for the individual mobile terminals is referred toas Unicast service. Since LTE differs from W-CDMA in that it does nothave dedicated channels (Dedicated Channel and Dedicated PhysicalChannel) for the individual mobile terminals in the transport channeland physical channel, it carries out the data transmission to theindividual mobile terminals through a shared channel.

An LTE communication system has two types of transmission modes in theE-MBMS service it provides: a multi-cell transmission mode and asingle-cell transmission mode. In the multi-cell transmission, aplurality of base stations transmit the same E-MBMS broadcast service atthe same frequency so that a mobile terminal can combine E-MBMS datasent from a plurality of base stations. The E-MBMS data is mapped ontoan MCH to be transmitted. On the other hand, in the single-celltransmission, the same E-MBMS broadcast service is transmitted withinonly one cell. In this case, the E-MBMS data is mapped onto a DL-SCH tobe transmitted. In the single-cell transmission, each base station cantransmit the E-MBMS data at a different frequency. To enable the mobileterminal to receive the multi-cell transmitted E-MBMS data and tocombine the E-MBMS data, it is necessary to suppress inter-symbolinterference which is interference between the E-MBMS data transmittedfrom a plurality of base stations. To handle the foregoing problem,Non-Patent Document 1 discloses a base station that transmits, at themulti-cell transmission, the E-MBMS data in such a manner that thetiming difference in the reception by a mobile terminal falls within anOFDM guard interval (referred to as “OFDM guard interval CP (CyclePrefix)”).

Non-Patent Document 1: 3GPP TR25.912 V7.0.0.

Although the LTE communication system employs OFDM as the access schemefor the downlink transmission, OFDM is considered to be an access schemethat is comparatively weak at interference. Thus, even if a plurality ofbase stations carry out multi-cell transmission that transmits the sameE-MBMS data, the number of the transmitting base stations is preferablydetermined at an appropriate number from the point of view of theinterference suppression. In addition, transmission from an unnecessarybase station not only brings about interference, but also is undesirablefrom the point of view of the effective use of radio resources.Non-Patent Document 2 discloses a communication method of transmittingthe E-MBMS data to a mobile terminal that sends a request only from abase station that receives from the mobile terminal the receptionrequest (counting or entry (such as entry, subscribe and activation))for the E-MBMS service (content) and from the neighboring base stationsof the base station. However, Non-Patent Document 2 does not describe amanner of selecting the base stations that transmit the E-MBMS data tothe mobile terminal that transmits the reception request.

Non-Patent Document 2: 3GPP R3-061205.

As a technique for a mobile terminal to receive and combine the samedata transmitted from a plurality of base stations, there is RAKEcombining at a soft handover. The RAKE combining is applied to acommunication system using the third-generation W-CDMA access scheme.When the mobile terminal is located close to a region at which the basestation is switched, a plurality of base stations in its adjacent cellstransmit the same dedicated data (DPDCH: Dedicated Physical DataChannel) to the mobile terminal. In the W-CDMA system, since each basestation multiplies a different scramble code, the mobile terminalcarries out receiving processing (despreading) separately for thereceived signal from each of a plurality of base stations to combine thedata from the plurality of base stations. For example, the mobileterminal which can receive the data from three base stations and performRAKE combining of them must have three branches of receiving sections(such as despreading sections) to perform receiving processing of thereceived data from each base station separately.

At the soft handover, an active set for the soft handover is created asa set of the base stations that transmit the same dedicated data to themobile terminal. As for the number of articles contained in the activeset, although it is variable according to the receiving capability ofthe mobile terminal (such as the number of base stations that canundergo receiving processing simultaneously) or according toinstructions from the communication system, its upper limit is set atsix in the present state of things. FIG. 13 is a diagram for explainingthe processing of creating the active set for the soft handover. In thegraph shown in FIG. 13, the vertical axis represents measurement qualityobtained by measuring by the mobile terminal the signal received fromthe base station, and the horizontal axis indicates time. The mobileterminal measures the powers of the received signals from first to thirdbase stations, thereby measuring reception qualities. At time T4, thecurve of the first base station crosses the curve of the second basestation, which means that the measurement quality of the second basestation exceeds the measurement quality of the first base station attime T4. Broken lines on FIG. 13, which represent the curve of the firstbase station before time T4 and the curve of the second base stationafter time T4, indicate the highest measurement quality among thereception qualities of the signals the mobile terminal is receiving fromthe plurality of base stations. In addition, dash-dotted lines, whichdenote a curve obtained from the curve indicating the highestmeasurement quality represented by the broken lines and a “reportingrange” of which the network side notifies the mobile terminal, representthe receiving level obtained by subtracting the reporting range from thevalues of the highest measurement quality (receiving level denoted bythe broken lines). The dash-dotted lines on the graph are used as adynamic threshold for creating the active set for the soft handover.

At time T1 in FIG. 13, the received signals from the first base stationand second base station show measurement qualities higher than thethreshold denoted by the dash-dotted lines. Accordingly, at time T1,both the first base station and second base station become the basestation candidates for the active set for the soft handover. On theother hand, at time T2, since the received signal from the third basestation exceeds the threshold denoted by the dash-dotted lines, thirdbase station is added as a new base station candidate for the active setfor the soft handover. Here, the mobile terminal transmits an additionalevent for making the third base station an additional candidate for theactive set. At time T3, since the received signal from the third basestation falls below the threshold denoted by the dash-dotted lines, thethird base station becomes a candidate to be deleted from the basestation candidates for the active set for the soft handover. Here, themobile terminal transmits a delete event for deleting the third basestation from the active set. As described above, the threshold formaking a decision of the additional or delete base station candidate isobtained by subtracting the reporting range from the highest measurementquality values. The highest measurement quality values vary inaccordance with the distance from the base station to the mobileterminal and the like. In other words, the threshold is a dynamicthreshold that varies in accordance with the receiving conditions of themobile terminal. As for the selection of the active set for the softhandover, Non-Patent Document 3 describes it.

Non-Patent Document 3: 3GPP TS25.331 V6.10.0.

Furthermore, a method and a system for performing a handoff by using thedynamic threshold that adaptively varies as described above aredescribed in Patent Document 1. Patent Document 1 discloses that theadaptive dynamic threshold is determined by a function of quality levelsof the highest transmission source and the lowest transmission source ofthe base stations (transmission sources) contained in the active set.

Patent Document 1: Japanese Patent Laid-Open No. 2003-525533.

DISCLOSURE OF THE INVENTION Problems to be Solved by the PresentInvention

Incidentally, deciding the base station(s) for transmitting the E-MBMSdata by using the method of creating the active set for the softhandover is considered to entail the following problems. As describedabove, the dedicated data the mobile terminal receives at the softhandover has passed through the spreading processing using a scramblecode peculiar to each base station, and the mobile terminal despreadsthe received signal from each base station using the scramble codepeculiar to the base station. Accordingly, the number of the basestations the mobile terminal can receive at the soft handover isrestricted by the receiving capability of the mobile terminal (such asthe number of the receiving sections for performing the despreading). Onthe other hand, in the case of E-MBMS, an LTE communication systemtransmits data using a common channel rather than a dedicated channel.In addition, it differs from the third-generation communication systemin that the transmission data does not undergo the spreading processingusing the scramble code peculiar to the base station. In other words,since a mobile terminal in the LTE communication system can simplycombine the E-MBMS data received from a plurality of base stations, thenumber of the base stations the mobile terminal can receive is notlimited.

A set of one or more base stations that transmit E-MBMS data to aparticular mobile terminal is referred to as “active set for E-MBMS” inthe following description for convenience sake. When the number of thebase stations contained in the active set for E-MBMS is low, there aresome cases where the mobile terminal cannot maintain the receptionquality of the E-MBMS data. On the other hand, an increasing number ofthe base stations will improve the reception quality of the E-MBMS databecause of the combining processing by the mobile terminal. However,transmission of the E-MBMS data from too many base stations is notappropriate from the viewpoint of making effective use of the radioresources. In the method of selecting base stations (method ofdetermining the active set for the soft handover) using the dynamicthreshold (dash-dotted line of FIG. 13) that is determined from therelationship between the reporting range of which the network sidenotifies the mobile terminal and the highest receiving level the mobileterminal measures, since the number of the base stations within thereporting range of FIG. 13 cannot be estimated, processing willsometimes become necessary for altering the reporting range depending onthe number of the base stations included in the active set for E-MBMS.However, if performing such processing, flexible and quick processing inresponse to variations in a radio environment cannot be expected.

Means for Solving Problems

The present invention is proposed to solve the foregoing problem, andprovides a data communication method executed in a communication systemincluding base stations for carrying out broadcast data transmission forproviding point-to-multipoint broadcast communication service using anOFDM (Orthogonal Frequency Division Multiplexing) scheme as a downlinkaccess scheme, and a mobile terminal for receiving and combining thebroadcast data transmitted from the plurality of base stations, the datacommunication method including: selecting processing of selecting one ormore base stations for transmitting the broadcast data to the mobileterminal, and of creating a set of the base stations for transmittingthe broadcast data, the selecting processing being executed by a servingbase station that arranges a transmission and reception schedule of themobile terminal; and receiving processing of receiving the broadcastdata transmitted from the one or more base stations selected by theselecting processing.

The present invention provides a mobile communication system includingbase stations for carrying out broadcast data transmission for providingpoint-to-multipoint broadcast communication service using an OFDM(Orthogonal Frequency Division Multiplexing) scheme as a downlink accessscheme, and a mobile terminal for receiving and combining the broadcastdata transmitted from the plurality of base stations, the mobilecommunication system being characterized by that the mobile terminalperforms candidate base station selecting processing including:processing of obtaining measurement quality values by measuringreception qualities of the one or more base stations from which themobile terminal receives the broadcast data; processing of making adecision on whether the measurement quality value of the serving basestation is higher than a threshold notified by the serving base station;processing of obtaining a sum measurement quality value by addingmeasurement quality values of other base stations to the measurementquality value of the serving base station until the sum measurementquality value satisfies reception quality defined by the threshold; andprocessing of notifying, when the sum measurement quality valuesatisfies the reception quality defined by the threshold, the servingbase station of the one or more base stations, whose signals arecombined with a signal received from the serving base station, ascandidate base stations; and the base station selects from the candidatebase stations notified by the mobile terminal one or more base stationsfor transmitting the broadcast data to the mobile terminal.

Advantages of the Present Invention

The data communication method in accordance with the present inventionincludes, in a communication system including base stations for carryingout broadcast data transmission for providing point-to-multipointbroadcast communication service using an OFDM (Orthogonal FrequencyDivision Multiplexing) scheme as a downlink access scheme and a mobileterminal for receiving and combining the broadcast data transmitted fromthe plurality of base stations, selecting processing of selecting one ormore base stations for transmitting the broadcast data to the mobileterminal, and of creating a set of the base stations for transmittingthe broadcast data, the selecting processing being executed by a servingbase station that arranges a transmission and reception schedule of themobile terminal; and receiving processing of receiving the broadcastdata transmitted from the one or more base stations selected by theselecting processing. Thus, it offers an advantage of being able toselect as an active set for E-MBMS an appropriate number of basestations that make it compatible to maintain good reception quality andto prevent interference (and to make efficient use of the radioresources).

The mobile communication system in accordance with the present inventionincludes base stations for carrying out broadcast data transmission forproviding point-to-multipoint broadcast communication service using anOFDM (Orthogonal Frequency Division Multiplexing) scheme as a downlinkaccess scheme, and a mobile terminal for receiving and combining thebroadcast data transmitted from the plurality of base stations, and inthe mobile communication system the mobile terminal performs candidatebase station selecting processing including: processing of obtainingmeasurement quality values by measuring reception qualities of the oneor more base stations from which the mobile terminal receives thebroadcast data; processing of making a decision on whether themeasurement quality value of the serving base station is higher than athreshold notified by the serving base station; processing of obtaininga sum measurement quality value by adding measurement quality values ofother base stations to the measurement quality value of the serving basestation until the sum measurement quality value satisfies receptionquality defined by the threshold; and processing of notifying, when thesum measurement quality value satisfies the reception quality defined bythe threshold, the serving base station of the one or more basestations, whose signals are combined with a signal received from theserving base station, as candidate base stations; and the base stationselects from the candidate base stations notified by the mobile terminalone or more base stations for transmitting the broadcast data to themobile terminal. Thus, it can select an appropriate number of basestations as the active set for E-MBMS, thereby offering an advantage ofbeing able to make it compatible to maintain good reception quality andto prevent interference (and to make efficient use of the radioresources).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a mobile communicationsystem in LTE;

FIG. 2 is a diagram showing an arrangement of channels used in an LTEcommunication system;

FIG. 3 is a block diagram showing a configuration of a mobile terminal;FIG. 4 is a block diagram showing a configuration of a base station;

FIG. 5 is a flowchart for explaining processing at a start of E-MBMSdata communication;

FIG. 6 is a diagram for explaining an idea of the processing ofselecting base station candidates for an active set for E-MBMS;

FIG. 7 is a flowchart for explaining processing of creating the activeset for E-MBMS;

FIG. 8 is a flowchart for explaining processing performed at a handover;

FIG. 9 is a flowchart for explaining processing of making a decisionwhether to carry out processing of creating the active set for E-MBMS;

FIG. 10 is a diagram for explaining an idea of the processing of makinga decision whether to carry out the processing of creating the activeset for E-MBMS;

FIG. 11 is a diagram showing a set of cells controlled by base stationsA-S;

FIG. 12 is a flowchart for explaining processing of creating the activeset for E-MBMS; and

FIG. 13 is a diagram for explaining processing of creating an active setfor a soft handover.

EXPLANATION OF SYMBOLS

-   1 aGW, 2 base station, 3 mobile station, 4 packet data network, 5    service center, 6 protocol processing section, 7 application    section, 8 transmission data buffer section, 9 encoder section, 10    modulating section, 11 frequency converting section, 12 antenna, 13    demodulating section, 14 decoding section, 15 control section, 16    aGW communication section, 17 other base station communication    section, 18 protocol processing section, 19 transmission data buffer    section, 20 encoder section, 21 modulating section, 22 frequency    converting section, 23 antenna, 24 demodulating section, 25 decoder    section, 26 control section.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a diagram showing a configuration of a mobile communicationsystem in LTE. In FIG. 1, an aGW 1 performs transmission and receptionof control data and user data with a plurality of base stations (eNodeB)2, and each base station 2 carries out transmission and reception ofdata with a plurality of mobile terminals 3. Between the base station 2and mobile terminal 3, are transmitted broadcast control information,information used for incoming call processing, dedicated control data,dedicated user data, and control data and user data for E-MBMS. Inaddition, communications between the base stations 2 have beeninvestigated. The base station 2 has an uplink and downlink scheduler.The scheduler enables the transmission and reception of the data betweenthe base station 2 and each mobile terminal 3, and arranges a schedulefor improving throughput of the individual mobile terminals 3 and of theentire mobile communication system.

The E-MBMS provides point to multipoint (P to M) broadcast communicationservice that transmits data from the base station to a plurality ofmobile terminals simultaneously. More specifically, information servicesuch as news and a weather forecast and broadcast mass service such asmobile TV have been investigated. The aGW 1 performs communication witha service center 5 via a PDN (Packet Data Network) 4. The service center5 is an apparatus for keeping and delivering contents for providing theE-MBMS service. A content provider transmits the E-MBMS data such asmobile TV broadcasting data to the service center 5. The service center5 stores the E-MBMS data, and transmits the E-MBMS data to the basestations 2 via the PDN 4 and aGW 1.

FIG. 2 is a diagram showing a channel arrangement. FIG. 2 shows mappingbetween logical channels and transport channels. The logical channelsare classified according to functions and logical characteristics of atransmission signal. The transport channels are classified according toa transmission mode. The broadcast control information is put on a BCCH(Broadcast Control Channel). The BCCH is mapped onto a BCH (BroadcastChannel), and transmitted from the base station to the mobile terminal.The information used for the incoming call processing is put on a PCCH(Paging Control Channel). The PCCH is mapped onto a PCH (PagingChannel), and transmitted from the base station to the mobile terminalswithin the cell. The dedicated control data addressed to the individualmobile terminals is put on a DCCH (Dedicated Control Channel).

In addition, the dedicated user data addressed to the individual mobileterminals are put on a DTCH (Dedicated Traffic Channel). The DCCH andDTCH are mapped onto a DL-SCH (Downlink Shared Channel), and transmittedfrom the base station to the individual mobile terminals separately.Conversely, using a UL-SCH (Uplink Shared Channel), the user data aretransmitted from the individual mobile terminals to the base stationseparately. The DL-SCH is a shared channel. The control data and userdata for the E-MBMS are put on an MCCH (Multicast Control Channel) andan MTCH (Multicast Traffic Channel), mapped onto the DL-SCH and MCH(Multicast Channel), respectively, and transmitted from the base stationto the mobile terminals. A connection request signal from each mobileterminal is transmitted from the mobile terminal to the base station viaa random access channel (RACH).

FIG. 3 is a block diagram showing a configuration of the mobileterminal. The transmission processing of the mobile terminal 3 iscarried out as follows. First, control data from a protocol processingsection 6 and user data from an application section 7 are stored in atransmission data buffer section 8. The data stored in the transmissiondata buffer section 8 is delivered to an encoder section 9 to undergoencoding processing such as error correction. Such data that is suppliedfrom the transmission data buffer section 8 to a modulating section 10directly without undergoing the encoding processing can also be present.The data passing through the encoding processing in the encoder section9 is subjected to modulating processing in the modulating section 10.The modulated data is converted to a baseband signal and then suppliedto a frequency converting section 11 to be converted to a radiotransmission frequency. After that, the transmission signal is sent froman antenna 12 to the base station 2. On the other hand, the receivingprocessing of the mobile terminal 3 is performed as follows. A radiosignal from the base station 2 is received by the antenna 12. Thereceived signal is converted from the radio receiving frequency to abaseband signal by the frequency converting section 11, and undergoesdemodulating processing by a demodulating section 13. The data after thedemodulation is delivered to a decoder section 14 which carries outdecoding processing such as error correction. Among the decoded data,the control data is delivered to the protocol processing section 6, andthe user data is delivered to the application section 7. A series of thetransmission and reception processing of the mobile terminal iscontrolled by a control section 15.

FIG. 4 is a block diagram showing a configuration of the base station.The transmission processing of the base station 2 is performed asfollows. An aGW communication section 16 carries out data transmissionand reception between the base station 2 and the aGW 1. One or moreother base stations communication section 17 performs data transmissionand reception with another base station. The aGW communication section16 and the one or more other base stations communication section 17 eachexchange information with a protocol processing section 18. The controldata from the protocol processing section 18 and the user data from theaGW communication section 16 and one or more other base stationscommunication section 17 are stored in the transmission data buffersection 19. The data stored in the transmission data buffer section 19are delivered to an encoder section 20 to undergo encoding processingsuch as error correction. Such data that is supplied from thetransmission data buffer section 19 to a modulating section 21 directlywithout undergoing the encoding processing can also be present. Theencoded data is subjected to modulating processing in the modulatingsection 21.

The modulated data is converted to a baseband signal, and then suppliedto a frequency converting section 22 to be converted to a radiotransmission frequency. After that, a transmission signal is transmittedfrom an antenna 23 to one or more mobile terminals 3. On the other hand,the receiving processing of the base station 2 is performed as follows.Radio signals from one or more mobile terminals 3 are received by theantenna 23. The received signal is converted from the radio receivingfrequency to a baseband signal by the frequency converting section 22,and a demodulating section 24 performs demodulating processing. Thedemodulated data is delivered to a decoder section 25 to undergodecoding processing such as error correction. Among the decoded data,the control data is delivered to the protocol processing section 18, andthe user data is delivered to the aGW communication section 16 and otherbase station communication section 17. A series of the transmission andreception processing of the base station 2 is controlled by a controlsection 26.

FIG. 5 is a flowchart for explaining processing at a start of E-MBMSdata communication. When a mobile terminal A uses an E-MBMS service, ittransmits an E-MBMS service entry to a serving base station (step 1).The serving base station is a base station that takes charge of uplinkand downlink scheduling of the mobile terminal, and that receives entry(Counting, or entry (such as Entry, Subscribe or Activation)) indicatinga reception request the mobile terminal transmits for receiving theE-MBMS service (content). Receiving the E-MBMS service entry transmittedfrom the mobile terminal A (step 2), the serving base station selects abase station for transmitting the E-MBMS data to the mobile terminal A.In other words, it performs selecting processing for selecting basestations included in the active set for E-MBMS of the mobile terminal A(step 3). The active set includes the serving base station itself.Details of selecting processing of selecting the active set for E-MBMSwill be described later. When the active set for E-MBMS is selected atstep 3, the base station that transmits the E-MBMS data to the mobileterminal A is decided. To request the selected base station to transmitthe E-MBMS data to the mobile terminal A, the serving base stationtransmits an addition request for the active set for E-MBMS to one ormore base stations included in the active set for E-MBMS at step 4. Atstep 6, a base station that receives the addition request for the activeset for E-MBMS from the serving base station performs the E-MBMS datatransmission to the mobile terminal A (step 7). At the same time, theserving base station starts the E-MBMS data transmission to the mobileterminal A as well (step 7). The mobile terminal A receives the E-MBMSdata transmitted from the serving base station and the base stations inthe active set for E-MBMS, and performs combining of the received data(step 8).

FIG. 6 is a diagram for explaining an idea of the processing ofselecting the candidate base stations for the active set for E-MBMS inthe mobile terminal. In FIG. 6, the vertical axis represents measurementquality when measuring the received signal from the base station, andthe horizontal axis represents time. FIG. 6 shows a bar chart indicatingmeasurement qualities from the serving base station A, and from basestations B and C other than the serving base station at individual timest1, t2, t3 and t4. As shown in FIG. 6, shading patterns of therespective base stations differ from each other in the bar chart. As fora threshold X, it is notified by the serving base station (or possiblyby the aGW or service center), and it is conceivable to transmit it fromthe serving base station to the mobile terminal via a logical channelsuch as a BCCH, DCCH, or MCCH. In addition, it is also conceivable totransmit it to the mobile terminal by mapping it onto a transportchannel such as a BCH, DL-SCH, or MCH. Although the threshold X is afixed threshold, such an arrangement is also possible in which theserving base station can vary it in accordance with a congestion levelor an amount of data transmitted by the E-MBMS service.

At time t1 of FIG. 6, bars indicating the measurement quality values ofthe serving base station A, base station B and base station C arearranged in descending order of the quality. In addition, it is shownthat all the measurement qualities of the serving base station A, basestation B and base station C are less than the threshold X. Thus, themobile terminal cannot achieve the reception quality the threshold Xrequires if it receives the E-MBMS data from one of the base stations A,B and C alone. However, the sum of the measurement quality value of theserving base station A and the measurement quality value of the basestation B exceeds the threshold X. Accordingly, the mobile terminal canachieve the reception quality the threshold X requires by receiving theE-MBMS data from the serving base station A and the E-MBMS data from thebase station B with the second highest measurement quality, and bycombining the two. In other words, to achieve the reception quality thethreshold X requires, there is no necessity to combine the E-MBMS datafrom all the base stations A-C, but it is enough to combine the E-MBMSdata from the base stations A and B. Thus, the mobile terminal selectsthe base station B as a candidate base station besides the serving basestation A that is always assigned as a member of the active set. At timet2 of FIG. 6, the measurement quality of the serving base station Aexceeds the threshold X. In other words, to achieve the receptionquality the threshold X requires, it is necessary to receive only theserving base station A without receiving the base station B or C.Accordingly, at time t2, the mobile terminal assigns only the servingbase station A to the candidate base stations for the active set forE-MBMS, and transmits a delete event for deleting the base station Bfrom the candidate base stations for the active set for E-MBMS.

At time t3 of FIG. 6, since the measurement quality of the serving basestation A is less than the threshold X, the mobile terminal must receivethe E-MBMS data from the base station B or C. At time t3, since the sumof the measurement quality value of the serving base station A and themeasurement quality value of the base station B exceeds the threshold Xas at time t1, the mobile terminal performs the processing of adding thebase station B to the candidate base stations for the active set forE-MBMS to receive the E-MBMS data from the base station B as well as theE-MBMS data from the serving base station A. In other words, the mobileterminal selects the serving base station A and the base station B asthe candidate base stations, and transmits to the serving base stationan additional event for adding the base station B, which is thecandidate base station, to the active set for E-MBMS. At time t4 of FIG.6, although it is the same as at time t3 that the measurement quality ofthe serving base station A is less than the threshold X, it differs fromthe case of time t3 in that the measurement quality of the serving basestation A is less than that of the base station C, and that themeasurement quality of the base station C is higher than the measurementquality of the base station B. In this case, since the sum of themeasurement quality value of the serving base station A and themeasurement quality value of the base station C exceeds the threshold X,the mobile terminal selects the serving base station A and base stationC as the candidate base stations to receive the E-MBMS data from thebase station C and serving base station A rather than from the basestation B. Then, the mobile terminal transmits to the serving basestation an update event (delete event of the base station B andadditional event of the base station C) for deleting the base station Bfrom the candidate base stations for the active set for E-MBMS and foradding the base station C thereto. The serving base station performs theselecting processing of selecting a base station to be assigned to theactive set for E-MBMS from one or more candidate base stations selectedby the candidate base station selecting processing executed by themobile terminal.

According to the candidate base station selecting processing and theactive set for E-MBMS selecting processing, the number of the basestations in the candidate base stations for the active set for E-MBMS istwo (serving base station A and base station B) at t1, one (serving basestation A) at t2, two (serving base station A and base station B) at t3,and two (serving base station A and base station C) at t4. Incidentally,although at time t4 the measurement quality value of the base station Cwhich is highest among the base stations other than the serving basestation A is added to the measurement quality value of the serving basestation A, and the sum is compared with the threshold X, the measurementquality value of the base station B can be added thereto to be comparedwith the threshold X. In other words, instead of adding a measurementquality value of a higher quality base station preferentially to themeasurement quality value of the serving base station A, it is alsopossible to add such processing as deciding a base station that has ameasurement quality value that will provide, when added to themeasurement quality value of the serving base station A, a resultant sumthat exceeds the threshold X and is closest to the threshold X.Employing the method can also achieve the advantage of the presentapplication, which makes it compatible to maintain good receptionquality and to prevent interference (and to make efficient use of theradio resources).

Here, referring to FIG. 6, the number of the base stations will bedescribed when using the method of creating the active set for a softhandover. As described above with reference to FIG. 13, the active setfor a soft handover supposes as a reference the dynamic threshold whichis obtained by subtracting the reporting range from the highestreception quality values among the reception qualities of neighboringbase stations the mobile terminal measures. In other words, the mobileterminal notifies the serving base station of all the base stationshaving the reception quality within a region determined by subtractingthe reporting range from the highest reception quality values as thecandidate base stations for the active set for a soft handover. Thecandidate base stations notified by the mobile terminal are addedformally to the active set if the serving base station authorizes. Thus,in FIG. 6, the candidate base stations become two (serving base stationA and base station B) at t1, two (serving base station A and basestation B) at t2, two (serving base station A and base station B) at t3,and three (serving base station A, base station B and base station C) att4. When creating the active set for E-MBMS in this way using the methodof creating the active set for a soft handover, it is found that thenumber of the base stations is excessive at t2 and t4. As for the E-MBMSdata transmission that employs OFDM which is weak at interference as thedownlink access scheme, it is preferable that the number of the basestations that transmit to the mobile terminal be as small as possible aslong as a certain level of the communication quality is ensured.Accordingly, it is inappropriate to apply the method of creating theactive set for a soft handover without change for creating the activeset for E-MBMS.

In the candidate base station selecting processing, the mobile terminalmeasures the reception quality of each base station. As a parameter ofthe reception quality, there is signal power or a signal to interferenceratio (SIR). However, to create an active set for E-MBMS at highaccuracy, it is recommended to measure the reception quality of adownlink reference symbol in the frequency band that actually transmitsthe E-MBMS data (regardless of whether the base station is transmittingthe E-MBMS service or not). This is because it is not affected byfrequency characteristics.

In addition, in the LTE system, the bandwidth of the base station isspecified that it is selected to be used from 1.25/2.5/5/10/15/20 MHz.The receivable bandwidth of the mobile terminal is specified in such amanner as to support a maximum of 20 MHz, and at least 10 MHz. When thereceivable bandwidth of the mobile terminal is less than the bandwidthof the base station, the mobile terminal negotiates with the basestation, and shifts between positions, that is, carries out processingof changing the center frequency of the radio transmission and receptionfrequency. The processing is referred to as retuning. Since the retuningimposes a heavy load on the scheduler of the mobile terminal and that ofthe serving base station, it is desirable not to execute it as much aspossible. However, when the E-MBMS data is transmitted at a positiondifferent from the position CUE position) at which the center frequencyof the mobile terminal is set, retuning becomes necessary to measure thequality of the E-MBMS data. Then, to measure the quality of the E-MEMSdata without retuning, the following first method and second method areconceivable.

The first method measures the downlink reference symbol at the UEposition, and considers the measurement result as the reception qualityin the frequency band in which the E-MBMS service is actuallytransmitted. The second method considers the result of measuring thereception quality of an SCH or BCH (transport channel) or BCCH (logicalchannel) the mobile terminal must receive independently of the E-MBMSdata reception as the reception quality in the frequency band in whichthe E-MBMS service is actually transmitted. Using the first method orthe second method makes it possible to measure the quality of thecandidate base stations to be included in the active set for E-MBMSwhile lightening the loads of the mobile terminal and serving basestation by reducing the retuning. When considering the result measuredby the first and second methods as the reception quality in thefrequency band that transmits the E-MBMS service, it is also possiblefor each base station to broadcast an offset value used at conversionthrough a BCH or the like. The measurement processing described above isimplemented by installing it in the demodulating section 13 in the blockdiagram of the mobile terminal shown in FIG. 3. In addition, the secondmethod is effective measurement processing even if the downlinkreference symbol separable for each base station is not transmitted inthe transmitted frequency band that actually transmits the E-MBMS data.

FIG. 7 is a flowchart for explaining the processing of creating theactive set for E-MBMS. The flowchart shown in FIG. 7 illustrates thecandidate base station selecting processing mainly executed by themobile terminal, and partly includes the active set for E-MBMS selectingprocessing in the serving base station. The mobile terminal selects acandidate for the base station to be included in the active set forE-MBMS, and notifies the serving base station of the candidate basestations. The serving base station selects the base stations belongingto the active set for E-MBMS from the candidate base stations notifiedby the mobile terminal, and decides them. In FIG. 7, the mobile terminalmeasures the reception qualities of neighboring base stations (step 1).At step 2, it arranges the measurement qualities of the neighboring basestations in descending order of measurement quality. For example, attime t1 of FIG. 6, the order becomes serving base station A, basestation B and base station C. At step 3, the mobile terminal comparesthe measurement quality of the serving base station A with the thresholdX. The serving base station A is always contained in the active set forE-MBMS regardless of the reception quality unless the serving basestation is changed. Accordingly, the measurement quality of the servingbase station A is selected at first as an item to be compared with thethreshold X. At step 3, if the measurement quality of the serving basestation A exceeds the threshold X as a result of comparing them (Yes atstep 3), the serving base station A can ensure enough quality for theE-MBMS data by itself. Thus, editing processing of the active set forE-MBMS at step 6 is executed.

If the measurement quality of the serving base station A is less thanthe threshold X (No at step 3), the serving base station A cannot ensureenough quality for the E-MBMS data by itself. Thus, at step 4, themeasurement quality value of the base station B, for example, is addedto the measurement quality value of the serving base station A. Then, atstep 5, the mobile terminal compares the sum measurement quality valueresulting from the addition with the threshold X. If the sum measurementquality value exceeds the threshold X (Yes at step 5), the active setediting processing at step 6 is executed. If the sum measurement qualityvalue of the serving base station A and base station B is still lessthan the threshold X (No at step 5), since the sum of the serving basestation A and base station B cannot ensure enough quality for the E-MBMSdata, the processing at step 4 and step 5 is further executed. Forexample, the measurement quality value of the base station C is added tothe sum measurement quality value of the serving base station A and basestation B, and its result is compared with the threshold X. If the summeasurement quality value of the serving base station A, base station Band base station C exceeds the threshold X (Yes at step 5), the activeset editing processing at step 6 is executed. The serving base stationis notified of the candidate base stations selected by executing theprocessing from step 1 to step 5, and executes the processing (editingprocessing of the active set for E-MBMS) at step 6, thereby deciding thebase stations to be included in the active set from the candidate basestations.

The E-MBMS active set editing processing at step 6 is processing ofmaking a decision as to whether it is necessary for one or more basestations that exceed the threshold X to be added to or deleted from theactive set for E-MBMS or the candidate base stations by comparing themwith the base stations included in the active set for E-MBMS or in thecandidate base stations at that point of time; and of requesting, if theaddition or deletion of the base stations is necessary, the serving basestation to update the active set for E-MBMS. For example, at step 3 ofFIG. 7, if the reception quality of the serving base station A exceedsthe threshold X (Yes at step 3), it is enough for the active set forE-MBMS to include only the serving base station A. In this case, if thecurrent active set for E-MBMS or the candidate base stations include anybase stations other than the serving base station A, the mobile terminaltransmits a request (delete event) to delete the base stations otherthan the serving base station A. Unless the current active set forE-MBMS or the candidate base stations include any base stations otherthan the serving base station A, no processing occurs. Likewise, at step5, if the sum reception quality of the serving base station A and basestation B exceeds the threshold X (Yes at step 5), it is enough for thecandidate base stations included in the active set for E-MBMS to includethe serving base station A and base station B. Accordingly, if thecurrent active set for E-MBMS or the candidate base stations include anybase stations other than the serving base station A or base station B,the mobile terminal transmits a request (delete event) to delete theunnecessary base station. On the contrary, unless the current active setfor E-MBMS includes the base station B, the mobile terminal transmits arequest (additional event) to add the base station B. Unless the currentactive set for E-MBMS or the candidate base stations include any basestations other than the serving base station A and base station B, noprocessing occurs.

A handover is performed when the mobile terminal moves from a certaincell to another cell. If the handover is performed, the serving basestation of the mobile terminal (assumed to be a mobile terminal A) isalso switched. When the mobile terminal A uses the E-MBMS service at thehandover, the active set for E-MBMS selected and updated by the servingbase station of the handover source undergoes new selection and creationby the serving base station of the handover target. FIG. 8 is aflowchart for explaining processing executed at the handover. In FIG. 8,the handover source base station transmits a handover request of themobile terminal A to the handover target base station (step 1). Inaddition, at step 1, it also notifies of the active set for E-MBMS ofthe current mobile terminal A. The handover target base station receivesthe handover request and the active set for E-MBMS transmitted from thehandover source base station (step 2). If the active set for E-MBMS forthe mobile terminal A is notified by the processing at step 1, thehandover target base station transmits an addition request for theactive set for E-MBMS to the base stations included in the active setfor E-MBMS at step 3. The handover target base station and base stationswhich receive the addition request for the active set for E-MBMS fromthe handover target base station prepare to transmit the E-MBMS data tothe mobile terminal A.

At step 4, the handover target base station transmits a signal notifyingof the authorization of the handover to the handover source basestation, and at step 5, the handover source base station receives it.After completing the foregoing processing, the serving base station isswitched from the handover source base station to the handover targetbase station at step 7, and the mobile terminal A receives the E-MBMSdata transmitted from the base stations in the new active set for E-MBMSthe handover target base station creates. On the other hand, since thehandover source base station completes the role as the serving basestation of the mobile terminal A, it performs the processing of deletingthe active set for E-MBMS of the mobile terminal A after succeeding inthe handover. After that, to select appropriate active set for E-MBMS,the handover target base station performs the processing of creating anddeciding the active set for E-MBMS of the mobile terminal A at step 6.Since the details of the processing at step 6 are the same as those ofthe processing described with reference to FIG. 7, their descriptionwill be omitted here.

As described above, as to the E-MBMS service in particular, which is thebroadcast service that transmits data from a plurality of base stationsto a mobile terminal in the LTE communication system that employs OFDM,an access scheme comparatively weak at interference, as the downlinkaccess scheme, it is important to set the number of the base stationsincluded in the active set for E-MBMS at an appropriate value to make itcompatible to maintain good reception quality and to preventinterference (and to make efficient use of the radio resources). Thepresent invention performs the candidate base station selectingprocessing including: (1) the processing in which the mobile terminalmeasures the reception quality of each base station and obtains themeasurement quality values; (2) the processing of making a decision asto whether the measurement quality value of the serving base stationexceeds the threshold X notified by the serving base station; (3) theprocessing of adding the measurement quality values of other basestations to the measurement quality value of the serving base stationuntil the resultant sum exceeds the threshold X notified by the servingbase station; and (4) the processing of notifying the serving basestation of the serving base station and other base stations included inthe sum measurement quality value that exceeds the threshold X as thecandidates for the active set for E-MBMS, thereby offering an advantageof being able to select as the active set for E-MBMS an appropriatenumber of base stations for making it compatible to maintain goodreception quality and to prevent interference (and to make efficient useof the radio resources).

In addition, instead of the mobile terminal, a certain apparatus on thenetwork side such as a base station and a base station control apparatuscan perform the candidate base station selecting processing. Forexample, the mobile terminal performs the processing of measuring thereception quality of each base station and obtaining the measurementquality values, and notifies an apparatus on the network side such asthe serving base station of the measurement result so that the servingbase station can perform: (2) the processing of making a decision as towhether the measurement quality value of the serving base stationexceeds the threshold X; (3) the processing of adding the measurementquality values of other base stations to the measurement quality valueof the serving base station until the resultant sum exceeds thethreshold X; and (4) the processing of deciding the serving base stationand other base stations included in the sum measurement quality valuethat exceeds the threshold X as the active set for E-MBMS. In this case,the subject of performing the candidate base station selectingprocessing and the subject of performing the active set for E-MBMSselecting processing by receiving the result of the candidate basestation selecting processing become the serving base station. This makesit possible to put the functions together.

Embodiment 2

In the embodiment 1, a description is made of the candidate base stationselecting processing including: (1) the processing in which the mobileterminal measures the reception quality of each base station and obtainsthe measurement quality values; (2) the processing of making a decisionas to whether the measurement quality value of the serving base stationexceeds the threshold X notified by the serving base station; (3) theprocessing of adding the measurement quality values of other basestations to the measurement quality value of the serving base stationuntil the resultant sum exceeds the threshold X notified by the servingbase station; and (4) the processing of notifying the serving basestation of the serving base station and other base stations included inthe sum measurement quality value that exceeds the threshold X as thecandidates for the active set for E-MBMS, and a description is made ofthe active set for E-MBMS selecting processing. In addition, adescription is made that the active set for E-MBMS selecting processingmentioned above is executed when the serving base station is switched asthe mobile terminal moves from a certain cell to another cell.

FIG. 9 is a flowchart for explaining the processing of making a decisionwhether to carry out the processing of creating the active set forE-MBMS. FIG. 10 is a diagram for explaining a concept of the processingof FIG. 9. The mobile terminal maintains good communication quality byreceiving and combining the data from the base stations included in theactive set for E-MBMS. At step 1, the mobile terminal measures thequality of the combined received signal acquired by receiving andcombining signals from a plurality of base stations, and obtains ameasurement quality value. As a parameter of the measurement quality ofthe E-MBMS data obtained by receiving and combining the data from theplurality of base stations, it is conceivable to use reception qualityof the E-MBMS data (power, SIR and the like) at the frequency and timethe E-MBMS data is really transmitted, or using a block error rate(BLER) of the E-MBMS data after decoding. Then, at step 2 and step 3,the mobile terminal compares the measurement quality value measured atstep 1 with two thresholds Z1 and Z2.

The thresholds Z1 and Z2, of which the serving base station (or possiblyaGW or service center) notifies the mobile terminal, can be transmittedthrough a logical channel such as BCCH, DCCH or MCCH, or can be mappedonto and transmitted through a transport channel such as BCH, DL-SCH orMCH. In addition, as the threshold X, the thresholds Z1 and Z2 can bevaried in accordance with the degree of congestion of the scheduler andthe amount of data transmitted by the E-MBMS service. The threshold Z1is the upper limit threshold to be compared with the measurement qualityvalues of E-MBMS data (of the combined received signal when combined).When the measurement quality values indicate that the quality is higherthan the level defined by the threshold Z1, a decision is made that theselecting processing is necessary to reduce the number of base stationsincluded in the active set for E-MBMS. The threshold Z2 is the lowestlimit threshold to be compared with the measurement quality values ofthe E-MBMS data. When the measurement quality values indicate that thequality is lower than the level defined by the threshold Z2, a decisionis made that the selecting processing is necessary to increase thenumber of base stations included in the active set for E-MBMS.

At step 2, if the measurement quality value is less than the thresholdZ1 (No at step 2), step 3 is executed. If the measurement quality valueis greater than the threshold Z1 (Yes at step 2), this means that themeasurement quality value indicates the quality higher than the leveldefined by the threshold Z1. Accordingly, a decision is made that thecurrent active set for E-MBMS is inappropriate. Thus, the processing atstep 1 and forward of FIG. 7 is executed so that the base stationcandidates for the active set for E-MBMS are altered in such a manner asto reduce the number of the base stations included in the active set forE-MBMS. However, such a case is also conceivable where no alteration ismade. As a concrete example, there is a case where if any one of thebase stations is deleted from the candidate base stations for the activeset for E-MBMS, the sum of the reception qualities does not exceeds thethreshold X. At step 3, if the measurement quality value is greater thanthe threshold Z2 (No at step 3) as a result of comparing the measurementquality value with Z2, the measurement quality of the E-MBMS data themobile terminal receives is in the range defined by the thresholds Z1and Z2. Accordingly, a decision is made that the selection of thecandidate base stations for the active set for E-MBMS is unnecessary. Ifthe measurement quality value is less than the threshold Z2 (Yes at step3), the measurement quality value does not reach the level defined bythe threshold Z1. Accordingly, a decision is made that the currentactive set for E-MBMS is inappropriate. Thus, the processing at step 1and forward of FIG. 7 is executed so that the active set for E-MBMS isaltered in such a manner as to increase the number of the base stationsincluded in the base station candidates for the active set for E-MBMS.

As shown in FIG. 10, it is found that the reception qualities of theE-MBMS data at time t1 and t2 are maintained at a certain leveldetermined by the threshold Z1 and threshold Z2. In other words, as forthe active set for E-MBMS at time t1 and t2, since the decision of No ismade at step 2 and step 3 of FIG. 9, the current active set for E-MBMSis accepted as appropriate. On the other hand, it is found that thereception quality of the E-MBMS data at time t3 is not at a certainlevel determined by the threshold Z1 and threshold Z2, but exceeds thelevel determined by the threshold Z1. In other words, as for the activeset for E-MBMS at time t3, since the decision of Yes is made at step 2of FIG. 9, the current active set for E-MBMS is considered to beinappropriate. Thus, in the case of time t3, since the reception qualityexceeds the level determined by the threshold Z1, revision is made insuch a manner that a base station is deleted from the current basestation candidates for the active set for E-MBMS.

As described above, as compared with the embodiment 1, the presentembodiment 2 can make a decision as to whether the candidate basestations for the active set for E-MBMS are appropriate or not from thereception quality of the E-MBMS data the mobile terminal is actuallyreceiving. In addition, it can reduce the number of executing theselecting processing of the candidate base stations for the active setfor E-MBMS (FIG. 7), thereby being able to reduce the processing load.

Embodiment 3

In the embodiment 2, if the quality of the received signal the mobileterminal forms by receiving and combining signals from a plurality ofbase stations is greater than the receiving level defined by thethreshold Z1 (Yes at step 2 of FIG. 9), or is less than the receivinglevel defined by the threshold Z2 (Yes at step 3 of FIG. 9), thedecision is made that the current base station candidates for the activeset for E-MBMS are inappropriate, and the selecting processing of theactive set for E-MBMS shown in FIG. 7 is executed. However, frequentlyperforming the candidate base station selecting processing as describedin the embodiment 1 has some fear of increasing the processing load ofthe mobile terminal.

In view of this, if the decision of Yes is made at step 2 or step 3 ofFIG. 9, the mobile terminal transmits to the serving base station anotification signal indicating simply “extra reception quality” or “badreception quality” instead of executing the candidate base stationselecting processing of FIG. 7. The signal notifying of “extra receptionquality” transmitted from the mobile terminal indicates that a certainlevel of the reception quality can be maintained even if the number ofthe base stations included in the active set for E-MBMS is reduced. Inthis case, the processing is performed of deleting the base stationgeographically remotest from the serving base station in the active setfor E-MBMS of the mobile terminal. On the other hand, the signalnotifying of “bad reception quality” transmitted from the mobileterminal indicates that a certain level of the reception quality cannotbe maintained unless the number of the base stations included in theactive set for E-MBMS is increased. In this case, the processing isperformed of adding the base station geographically closest to theserving base station in the base stations not included in the active setfor E-MBMS of the mobile terminal. Adopting the method as describedabove can prevent the mobile terminal from executing the processing ofselecting the candidate base stations, thereby being able to reduce theprocessing load of the mobile terminal.

Embodiment 4

As a result of measuring the quality of SCH by the mobile terminal, ifthe measurement quality of the base station C exceeds the measurementquality of the serving base station A as at time t4 of FIG. 6, ahandover is usually performed to the base station C. In this case, theserving base station is changed from the base station A to the basestation C. However, if the base station C is congested, there are somecases that are unsuitable for the handover because the load of itsscheduler is very heavy. In such a case, as a handover target, the basestation B that is not so busy as the base station C can be considered tobe more appropriate though it is lower than the base station C in thereception quality of SCH.

As for the base station that will become a handover target in thefuture, it is preferable that it be included in the active set forE-MBMS before the handover to enable the mobile terminal to receive theE-MBMS data without any interruption at the handover. Thus, at time t4of FIG. 6, the mobile terminal can select the base station C as a basestation candidate for the active set for E-MBMS, considering that ahandover can be made to the uncongested base station B in the futurerather than to the congested base station C although the measurementquality of the base station B is inferior to the measurement quality ofthe base station C. Such a configuration makes it possible, in additionto the advantages of the foregoing embodiment 1, to select the activeset for E-MBMS in accordance with the conditions of the mobilecommunication system such as the degree of congestion of the basestation.

Embodiment 5

The embodiments 1-4 perform the candidate base station selectingprocessing including: (1) the processing of measuring the receptionquality of each base station and obtaining the measurement qualityvalues; (2) the processing of making a decision as to whether themeasurement quality value of the serving base station exceeds thethreshold X notified by the serving base station; (3) the processing ofadding the measurement quality values of other base stations to themeasurement quality value of the serving base station until theresultant sum exceeds the threshold X notified by the serving basestation; and (4) the processing of notifying the serving base station ofthe serving base station and other base stations included in the summeasurement quality value that exceeds the threshold X as the candidatesfor the active set for E-MBMS, and receiving the result, the servingbase station selects and determines the active set for E-MBMS. However,it is certainly one idea to decide the base stations to be included inthe active set for E-MBMS in advance in accordance with the serving basestation.

FIG. 11 is a diagram showing a set of cells the base stations A-Smanage. In FIG. 11, when the base station A is the serving base station,for example, the base stations B, C, D, E, F and G adjacent to theserving base station A are automatically decided as the active set forE-MBMS. The base stations A-G transmit the E-MBMS data to the mobileterminal. In addition, when the base station G is the serving basestation, the base stations B, A, F, K, J and I adjacent to the servingbase station G are decided as the active set for E-MBMS. Thecorrespondence between the serving base station and the base stationsincluded in the active set for E-MBMS can be kept in the form of a list.After receiving an entry for the E-MBMS service from the mobileterminal, the serving base station refers to the list, and decides theactive set for E-MBMS for the mobile terminal that makes the entry.

Instead of deciding the base stations to be included in the active setfor E-MBMS in advance in accordance with the serving base station, it isalso possible to select them in accordance with conditions of the entiremobile communication system (such as the degree of congestion and theamount of data transmitted by the E-MBMS service). It can be implementedby providing several types of lists of eligible base stations decided inadvance. For example, consider a handover (switching of the serving basestation) when the mobile communication system is congested as a whole.Even in the case where the handover is to be made from the handoversource base station (base station A) to its adjacent base station (basestation B, for example) properly, if the base station B is congested(the load of its scheduler is very heavy), it is conceivable to performthe handover to the base station H which is not so busy as the basestation B though inferior in the quality. To enable the mobile terminalto receive the E-MBMS data without interruption at the handover in theforegoing conditions, the handover target (base station H) must beincluded in the active set for E-MBMS before the handover.

FIG. 12 is a flowchart for explaining processing of creating the activeset for E-MBMS. At step 1, receiving an E-MBMS entry from the mobileterminal (Yes at step 1), the serving base station makes a decision asto whether the mobile communication system is congested. If the mobilecommunication system is congested (Yes at step 2), the more the numberof the candidate base stations for the handover is, the better. Thus,the serving base station employs the list B of FIG. 12( b), and selectsthe base stations A-S as the active set for E-MBMS. On the other hand,when the mobile communication system is not congested (No at step 2),the serving base station makes a decision at step 3 on whether the datarate of the E-MBMS exceeds a threshold. The threshold is used forselecting the list A or list B, and when the data rate is relativelyhigh, the list B is employed. Thus, if the data rate exceeds thethreshold (Yes at step 3), the serving base station employs the list Bof FIG. 12( b), and selects the base stations A-S as the active set forE-MBMS. On the other hand, unless the data rate exceeds the threshold(No at step 3), it employs the list A, and selects the base stations A-Gas the active set for E-MBMS.

As described above, concerning the entry from the mobile terminal, theactive set for E-MBMS is selected by referring to the list of theneighboring base stations determined in advance. This offers anadvantage of being able to select the appropriate active set for E-MBMSquickly after the serving base station receives the E-MBMS service entryfrom the mobile terminal. In addition, as compared with the methoddescribed in the embodiment 1, since it is unnecessary for the mobileterminal to notify the serving base station of any information otherthan the E-MBMS entry (such as the notification of the measurementresult from the mobile terminal to the serving base station), thepresent embodiment 5 offers an advantage of being able to make effectiveuse of the radio resources. Furthermore, the active set for E-MBMS canbe selected by referring to the list of eligible base stations decidedin advance. This offers an advantage of being able to obviate the needfor performing processing of selecting appropriate base stations as theactive set for E-MBMS, thereby being able to reduce the load of theserving base station. Moreover, since it is not necessary to measure thereception quality from the base stations for selecting the active setfor E-MBMS, or to notify the serving base station of the measurementresult, there is an advantage of being able to reduce the load of themobile terminal. In addition, it becomes possible to select the activeset for E-MBMS suitable for the conditions of the mobile communicationsystem. As an example of a concrete advantage, there is an advantage ofbeing able to receive the E-MBMS data without interruption at thehandover independently of the degree of congestion of the mobilecommunication system.

As for the processing executed at the handover, it is described inconnection with the embodiment 1. It is possible for the embodiment 5 toperform the same handover processing as in the embodiment 1. In thiscase, however, although in the embodiment 1 the serving base stationselects the active set for E-MBMS by using the candidate base stationsfor the active set for E-MBMS obtained from the measurement result bythe mobile terminal, in the embodiment 5 the serving base stationselects the active set for E-MBMS by referring to the lists withoutusing the candidate base stations for the active set for E-MBMS by themobile terminal. Accordingly, the handover target base station candecide the active set for E-MBMS of the handover target base station byreferring to the lists immediately after receiving the handover requestat step 2 of FIG. 8. This makes it possible to omit step 6 of FIG. 8. Inaddition, the present embodiment 5 can achieve an advantage of beingable to decide the optimum base stations to be included in the activeset for E-MBMS in the handover target base station quickly.

1. A data communication method executed in a communication systemincluding base stations for carrying out broadcast data transmission forproviding point-to-multipoint broadcast communication service using anOFDM (Orthogonal Frequency Division Multiplexing) scheme as a downlinkaccess scheme, and a mobile terminal for receiving and combining thebroadcast data transmitted from the plurality of base stations, saiddata communication method comprising: selecting processing of selectingone or more base stations for transmitting the broadcast data to saidmobile terminal, and of creating a set of said base stations fortransmitting the broadcast data, said selecting processing beingexecuted by a serving base station that arranges a transmission andreception schedule of said mobile terminal; and receiving processing ofreceiving the broadcast data transmitted from said one or more basestations selected by the selecting processing.
 2. The data communicationmethod according to claim 1, including candidate base station selectingprocessing executed by said mobile terminal, said candidate base stationselecting processing comprising: processing of obtaining measurementquality values by measuring reception qualities of said one or more basestations from which said mobile terminal receives the broadcast data;processing of making a decision on whether the measurement quality valueof said serving base station is higher than a threshold notified by saidserving base station; processing of obtaining a sum measurement qualityvalue by adding measurement quality values of other base stations to themeasurement quality value of said serving base station until the summeasurement quality value satisfies reception quality defined by thethreshold; and processing of notifying, when the sum measurement qualityvalue satisfies the reception quality defined by the threshold, saidserving base station of the one or more base stations, whose signals arecombined with a signal received from said serving base station, ascandidate base stations.
 3. The data communication method according toclaim 2, wherein said mobile terminal is notified of a first thresholdand a second threshold, said first threshold defining an upper limitreception quality of a combined received signal obtained by combiningsignals received from one or more base stations by said mobile terminal,and said second threshold defining a lowest limit reception quality ofthe combined received signal; and the candidate base station selectingprocessing is executed when reception quality of the combined receivedsignal is higher than the first threshold, or when the reception qualityof the combined received signal does not satisfy the reception qualitythe second threshold requires.
 4. The data communication methodaccording to claim 2, wherein the candidate base station selectingprocessing is executed at least at a handover that changes the servingbase station for arranging a schedule of said mobile terminal.
 5. Thedata communication method according to claim 1, wherein said mobileterminal is notified of a first threshold and a second threshold, saidfirst threshold defining an upper limit reception quality of a combinedreceived signal obtained by combining signals received from one or morebase stations by said mobile terminal, and said second thresholddefining a lowest limit reception quality of the combined receivedsignal; and said mobile terminal performs notification processing inwhich when the reception quality of the combined received signal ishigher than the first threshold, said mobile terminal notifies saidserving base station that the reception quality is too high, and whenthe reception quality of the combined received signal does not satisfyreception quality the second threshold requires, said mobile terminalnotifies said serving base station that the reception quality isinsufficient.
 6. The data communication method according to claim 1 or5, wherein said one or more base stations for transmitting the broadcastdata to said mobile terminal are determined in advance concerning saidserving base station, and said selecting processing selects one or morecorresponding base stations in accordance with said serving basestation.
 7. A mobile communication system including base stations forcarrying out broadcast data transmission for providingpoint-to-multipoint broadcast communication service using an OFDM(Orthogonal Frequency Division Multiplexing) scheme as a downlink accessscheme, and a mobile terminal for receiving and combining the broadcastdata transmitted from the plurality of base stations, said mobilecommunication system being characterized by that: said mobile terminalperforms candidate base station selecting processing comprising:processing of obtaining measurement quality values by measuringreception qualities of said one or more base stations from which saidmobile terminal receives the broadcast data; processing of making adecision on whether the measurement quality value of said serving basestation is higher than a threshold notified by said serving basestation; processing of obtaining a sum measurement quality value byadding measurement quality values of other base stations to themeasurement quality value of said serving base station until the summeasurement quality value satisfies reception quality defined by thethreshold; and processing of notifying, when the sum measurement qualityvalue satisfies the reception quality defined by the threshold, saidserving base station of the one or more base stations, whose signals arecombined with a signal received from said serving base station, ascandidate base stations; and said base station selects from saidcandidate base stations notified by said mobile terminal one or morebase stations for transmitting the broadcast data to said mobileterminal.